The present invention generally relates to dc voltage converters and particularly relates to an inverting converter operating under hysteretic feedback control, such as used for dc-to-dc conversion or dc-to-RF modulation.
Switch-mode dc-to-dc converters offer potentially significant efficiency gains as compared to their linear converter counterparts, particularly for applications involving high load currents and/or large conversion voltage differentials. As with most things, however, the switch-mode converter""s list of advantages has a corresponding list of disadvantages. Potential disadvantages include poor transient response, possible operational instability, significant radiated and conducted electromagnetic interference (EMI), and the need for high performance components, e.g., inductors with good core saturation properties and low dc losses, low equivalent series resistance (ESR) capacitors, etc.
Careful printed circuit board (PCB) layout, such as by minimizing switched current loop areas and using appropriate grounding layouts, and careful component selection, together go a long way toward addressing many of the potential disadvantages associated with switch-mode converters. More fundamentally, however, some converter topologies offer intrinsically superior performance, although it should be understood that a particular converter topology""s xe2x80x9csuperiorityxe2x80x9d might apply only in the context of particular applications or uses.
The xe2x80x9c{dot over (C)}uk converterxe2x80x9d represents one such converter topology that offers superior switch-mode performance for an appreciable variety of applications. Developed by Dr. Slobodon {dot over (C)}uk, and described in exemplary fashion in U.S. Pat. No. 4,184,197 (now expired), the xe2x80x9c{dot over (C)}uk topologyxe2x80x9d offers particular advantages with regard to EMI in that neither its input nor its output currents are truly xe2x80x9cswitchedxe2x80x9d on and off. Many other converter topologies electrically switch (connect/disconnect) one or both the input and output converter circuits, resulting in pulsed input or output currents, or both. For example, so-called xe2x80x9cboostxe2x80x9d and xe2x80x9cbuck-boostxe2x80x9d converters have at least one side (input or output) with pulsed current. Pulsed input/output current increases the risk of EMI problems, among other things.
With the {dot over (C)}uk topology, an input inductor circuit is coupled to an output inductor circuit through a storage element, which typically is implemented as a charge storage capacitor. The opposing sides of that capacitor are alternately switched to reference ground, such that the desired output voltage is generated as a function of charge transfer through the switched capacitor. With the series placement of the input and output inductors, the input and output currents are naturally xe2x80x9csmoothedxe2x80x9d rather than pulsed, which yields greatly reduced EMI.
Because the {dot over (C)}uk topology is naturally inverting, although that behavior may be altered through modified capacitor switching, it stands as a natural candidate for use in environments with readily available negative voltage supplies, such as the xe2x88x9248 VDC xe2x80x9crailxe2x80x9d commonly used in telecommunication circuits, e.g., in Central Office systems, and in wireless communication base stations. However, in many such circumstances, such as where the negative rail otherwise might be xe2x80x9ctappedxe2x80x9d for use in high-power radio frequency transmit signal generation, the performance of the general {dot over (C)}uk topology falls short in terms of bandwidth, as well as in other areas.
Ideally, then, one would modify the general {dot over (C)}uk topology to extend its performance in the context of telecommunication usage. With the required improvements, an appropriately modified {dot over (C)}uk converter would offer an advantageous means for using the reliable and ubiquitous negative supply rail(s) available in telecommunication and wireless network systems directly in transmit signal generation, or for other demanding, relatively high-bandwidth voltage conversion tasks.
The present invention comprises a method and apparatus to extend the operation of an inverting {dot over (C)}uk converter by applying closed-loop hysteretic control. Using such hysteretic control expands the range of advantages of the general {dot over (C)}uk converter topology to include significantly enhanced line and load regulation performance. That is, hysteretic feedback control is used to extend the control bandwidth of the {dot over (C)}uk converter topology in a dc voltage converter, enabling such converters to serve in dc-to-RF modulation applications and other radio amplifier applications, microprocessor supply controller applications and other higher-performance dc-to-dc voltage conversion applications.
In an exemplary embodiment, a dc voltage converter includes a hysteretically controlled {dot over (C)}uk converter circuit and an optional damping control circuit to ensure stability of operation over the extended converter bandwidth. Advantageously, the damping control circuit may be implemented at low component cost as a resistor-capacitor xe2x80x9csnubberxe2x80x9d circuit coupled across the energy transfer capacitor used to couple input and output inductive circuits that form part of the basic {dot over (C)}uk converter topology. Where the inventive converter is used in RF signal amplification, the damping circuit may be tuned for sub-harmonics of the RF signal.
Further, the feedback control loop of the converter may include, in addition to a hysteretic controller, a pulse controller that limits the maximum on/off pulse time. With inclusion of such pulse limiting, a transformer drive circuit may be used without the potential core saturation problems that might otherwise arise. Use of the transformer drive circuit may be particularly desirable where the input voltage to the converter has a fairly high magnitude, and thus would complicate the design of non-isolated drive circuits. One example of such circumstances is usage of the converter as a supply signal modulator for a power amplifier in a xe2x80x9cpolar modulationxe2x80x9d radio transmitter circuit within a wireless communication base station. In such applications, the switching rate of the inventive converter may be configured based on known RF signal characteristics, such as the information symbol or chip rate of the signal.
Wireless base stations typically provide a high-reliability negative supply rail at xe2x88x9248 VDC or some other standard voltage, as do many other types of telecommunication equipment. A supply signal modulator for a polar modulation transmitter thus might comprise a linear amplifier in combination with a dc voltage converter configured according to the present invention. The linear amplifier advantageously would have a relatively high bandwidth and supply the higher frequency but lower power components of the modulated supply signal, while the converter of the present invention would supply the lower frequency but higher power components of that signal. With that configuration, much of the power for supply signal modulation would be taken directly from the highly reliable xe2x88x9248 VDC rail, thus simplifying the circuit design.
Most radio base station high power RF amplifier circuits first convert xe2x88x9248 VDC to +28 VDC, but the present invention eliminates the need for that step. That is, RF signal power may be taken directly and efficientlyxe2x80x9d from the xe2x88x9248 VDC rail using the dc voltage converter of the present invention.
Of course, those skilled in the art will appreciate that the present converter""s extended bandwidth, stability of control, and convenient transformer drive capability make it an exemplary candidate for use in a wide range of applications. As such, it should be understood that the present invention is not limited by the following exemplary details.